Shock wave electrodes

ABSTRACT

Disclosed herein shock wave catheters comprising one or more shock wave electrodes for cracking calcifications located within blood vessels. In some variations, a shock wave catheter has first and second shock wave electrodes each circumferentially disposed over the outer surface of the catheter. In certain variations, the first electrode has a recess and the second electrode has a protrusion that is received by the recess and a spark gap is located along the separation between the recess and the protrusion. The second electrode can also have a recess that receives a protrusion from a third shock wave electrode, where the separation between the second and third electrodes along the separation between the recess and the protrusion forms a second spark gap. A shock wave can be initiated across these spark gaps when a voltage is applied over the electrodes.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. Provisional Patent Application Ser. No. 62/257,141 entitled “SHOCK WAVE ELECTRODES,” filed on Nov. 18, 2015, which is hereby incorporated by reference in its entirety for all purposes.

FIELD

The present disclosure relates generally to shock wave electrodes, and, more specifically, to electrodes for the generation of shock waves within vascular structures.

BACKGROUND

Electrohydraulic lithotripsy has been typically used for breaking calcified deposits or “stones” in the urinary or biliary track. Recent work by the assignee shows that lithotripsy electrodes may similarly be useful for breaking calcified plaques in the wall of a vascular structure. Shock waves generated by lithotripsy electrodes may be used to controllably fracture a calcified lesion to help prevent sudden stress and injury to the vessel or valve wall when it is dilated using a balloon. A method and system for treating stenotic or calcified vessels is described in U.S. Pat. Nos. 8,956,371, 8,888,788, and 9,011,463 incorporated herein by reference. A method and system for treating stenotic or calcified aortic valves is described in U.S. Pat. No. 9,044,618 incorporated herein by reference. As described in those applications, a balloon is placed adjacent leaflets of a valve or vessel to be treated and is inflated with a liquid. Within the balloon are one or more shock wave electrodes that produce shock waves that propagate through the liquid and impinge upon the valve or vessel. The impinging shock waves soften, break and/or loosen the calcified regions for removal or displacement to open the valve or vessel (e.g., to enlarge the valve opening or clear the lumen of the vessel).

In electrode-based lithotripsy systems, a pair of electrodes is used to generate a shock wave. When a high voltage is applied across the electrode pair, a plasma arc forms between them, giving rise to a steam bubble in the fluid. A first shock wave occurs when the steam bubble first forms and a second shock wave occurs as the steam bubble collapses. The timing and size of the bubble, along with the sonic output and propagation direction of the resultant shock waves, depend at least in part on the location, geometry and size of the electrodes. The size and arrangement of the electrodes also impact the types of vascular structures that may be accessed and treated by the shock wave catheter. For example, coaxial shock wave electrode pairs can comprise an inner wire that is inserted within an outer cable, where the conductive portions of each are exposed at the tips. In this arrangement, a high voltage applied across the inner wire and outer cable gives rise to a shock wave that propagates away from the exposed tips of the inner wire and outer cable. Additional improved lithotripsy or shock wave electrodes that can readily access and generate sufficient sonic output to treat various locations in the vasculature for angioplasty and/or valvuloplasty procedures may be desirable.

BRIEF SUMMARY

Described herein are electrodes for the generation of shock waves within vascular structures. One variation of a shock wave electrode pair comprises a first electrode that is circumferentially disposed over an outer surface of an elongate member and a second electrode also circumferentially disposed over the outer surface of the elongate member, where a spark gap is formed at the narrowest separation distance between the two electrodes. The first and second electrodes may be coplanar along the surface of the elongate member (e.g., located along a single layer). In some variations, the first electrode may comprise a recess and the second electrode may comprise a projection that is located within the recess such that the separation between the projection and the recess form the spark gap. Each electrode may comprise a portion that is covered by an insulating layer or coating and a portion that is not covered by an insulating layer or coating so that the electrically conductive substrate of the electrode is exposed. A catheter may have one or more of these electrode pairs arranged in series. In use, a voltage generator may be provided and a first wire may connect the proximal-most electrode to a positive terminal of the voltage generator and a second wire may connect the distal-most electrode to a negative terminal of the voltage generator, and a high voltage may be applied across the positive and negative terminals in order to generate a series of shock waves along the length of the catheter at each of the electrode pairs.

One variation of a shock wave catheter may comprise an axially extending elongate member, a first electrode circumferentially disposed over an outer surface of the elongate member, the first electrode comprising a recess along an edge of the first electrode, and a second electrode circumferentially disposed over the outer surface of the elongate member and adjacent to the first electrode, the second electrode comprising a projection along the edge of the second electrode that is received by the recess. A spark gap may be formed by a separation between the projection and the recess. The catheter may be configured such that when a voltage is applied across the first and second electrodes, a current may flow across the spark gap between the first electrode and the second electrode such that a shockwave is initiated at the spark gap. Some systems may further comprise a voltage source and a first wire that connects the first electrode to a positive terminal of the voltage source, and a second wire connects the second electrode to a negative terminal of the voltage source. The first wire and the second wire may be located on the outer surface of the elongate member. The axially extending elongate member may comprise a guide wire lumen extending therethrough. Some shock wave catheter variations may further comprise a third electrode circumferentially disposed over the outer surface of the elongate member and adjacent to the second electrode, where the second electrode may comprise a recess and the third electrode may comprise a projection that is received by the recess of the second electrode, and where a space between the projection of the third electrode and the recess of the second electrode may form a second spark gap such that when a voltage is applied across the first and third electrodes, a current may flow across the first spark gap to initiate a first shock wave and across the second spark gap to initiate a second shock wave.

In some variations, the shock wave catheter may further comprise a fourth electrode circumferentially disposed over the outer surface of the elongate member and adjacent to the third electrode, and a fifth electrode circumferentially disposed over the outer surface of the elongate member and adjacent to the fourth electrode. The fourth electrode may comprise a projection that is received by a recess of the third electrode, and a space between the projection of the fourth electrode and the recess of the third electrode may form a third spark gap. The fourth electrode may also comprise a recess that receives a projection of the fifth electrode, and a space between the recess of the fourth electrode and the protrusion of the fifth electrode may form a fourth spark gap so that when a voltage is applied across the first and fifth electrodes, a current may flow across the first spark gap to initiate a first shock wave, across the second spark gap to initiate a second shock wave, across the third spark gap to initiate a third shock wave, and across the fourth spark gap to initiate a fourth shock wave. Optionally, some shock wave catheters may further comprise a sixth electrode circumferentially disposed over the outer surface of the elongate member and adjacent to the fifth electrode, and a seventh electrode circumferentially disposed over the outer surface of the elongate member and adjacent to the sixth electrode. The sixth electrode may comprise a projection that is received by a recess of the fifth electrode, and a space between the projection of the sixth electrode and the recess of the fifth electrode may form a fifth spark gap. The sixth electrode may further comprise a recess that receives a projection of the seventh electrode, and a space between the recess of the sixth electrode and the protrusion of the seventh electrode may form a sixth spark gap such that when a voltage is applied across the first and seventh electrodes, a current may flow across the first spark gap to initiate a first shock wave, across the second spark gap to initiate a second shock wave, across the third spark gap to initiate a third shock wave, across the fourth spark gap to initiate a second shock wave, across the fifth spark gap to initiate a fifth shock wave, and across the sixth spark gap to initiate a sixth shock wave.

Any of the electrode pairs described herein may comprise a first electrode having a recess and a second electrode adjacent to and co-planar with the first electrode and having a protrusion and the recess may have a concave curve and the protrusion may have a convex curve that corresponds with the concave curve. An electrode may comprise a proximal end, a distal end, and a spiral body therebetween. The spiral body may comprise one or more helices that wrap along the outer surface of the elongate body.

Another variation of a shock wave catheter may comprise an axially extending elongate member, a first electrode circumferentially disposed over an outer surface of the elongate member, the first electrode comprising a first electrically conductive region having a first surface area and a second electrode circumferentially disposed over the outer surface of the elongate member that is coplanar and adjacent to the first electrode. The second electrode may comprise a second electrically conductive region having a second surface area that is greater than the first surface area. A spark gap may be formed by a separation between the conductive region of the first electrode and the conductive region of the second electrode such that when a voltage is applied across the first and second electrodes, a current may flow across the spark gap between the first electrode and the second electrode such that a shockwave is initiated at the spark gap. Some variations may further comprise a voltage source, and a first wire may connect the first electrode to a positive terminal of the voltage source, and a second wire may connect the second electrode to a negative terminal of the voltage source, and the first wire and the second wire may be located on the outer surface of the elongate member. The axially extending elongate member may comprise a guide wire lumen extending therethrough. In some variations, the conductive region of the first electrode may have a first arcuate portion and the conductive region of the second electrode may have a second arcuate portion that is complementary to the first arcuate portion. For example, the first arcuate portion may comprise a convex curve and the second arcuate portion may comprise a concave curve. Alternatively or additionally, the first arcuate portion may comprise a concave curve and the second arcuate portion may comprise a convex curve. The radius of curvature of the first arcuate portion may be larger than the radius of curvature of the second arcuate portion, or alternatively, the radius of curvature of the first arcuate portion may be less than the radius of curvature of the second arcuate portion. The first arcuate portion may comprise a circular protrusion and the second arcuate portion may comprise a circular groove. Alternatively, the first arcuate portion may comprise a circular groove and the second arcuate portion may comprise a circular protrusion. A ratio of the surface area of the first conductive region to the surface area of the second conductive region may be from about 1:2 to about 1:20, e.g., from about 1:4 to about 1:10. The second electrode may comprise a proximal end, a distal end, and an electrically conductive body extending therebetween. The electrically conductive body may comprise one or more flexible spirals and/or one or more flexible helices. The electrically conductive body may comprise a flexible twisted portion.

Other variations of a shock wave catheter may comprise an axially extending elongate member, a first ring electrode mounted circumferentially around the elongate member, a second ring electrode mounted circumferentially around the elongate member and positioned adjacent to the first ring electrode, and a third ring electrode mounted circumferentially around the elongate member and positioned adjacent to the first ring electrode. A first spark gap may be defined between the first and second ring electrodes and a second spark gap may be defined between the second and third ring electrodes. Each spark gap may include an arcuate recess formed along a side edge of one of the ring electrodes and a complimentary arcuate projection formed along the side edge of the adjacent ring electrode, where the projection may fit into the associated recess. The first spark gap may be circumferentially offset from the second spark gap. The first and third ring electrodes may be connectable to a high voltage source. The second ring electrode may be a non-insulated metal and the recess of the first and second spark gaps may be formed in the second ring electrode. The projection of the first spark gap may be formed in the first ring electrode and the projection of the second spark gap may be formed in the third ring electrode, where the first and second ring electrodes may have an insulating coating except in the region of the projections. The surface area of the second ring electrode may be greater than the surface area of each of the projections of the first and third ring electrodes. The first electrode and the third electrode may each have a proximal portion, a distal portion, and a body therebetween, where the body may extend longitudinally along the axially extending elongate member and may comprise a spiral. In some variations, the body may comprise a helix, and/or one or more turns around the surface of the elongate member.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A depicts one variation of shock wave catheter comprising single-layer electrodes.

FIG. 1B depicts one variation of a single-layer electrode. FIG. 1C depicts one variation of a pair of single-layer electrodes. FIG. 1D depicts another variation of a pair of single-layer electrodes.

FIG. 2A depicts one variation of single-layer electrodes of a shock wave catheter that interfit with each other. FIG. 2B depicts another variation of single-layer electrodes of a shock wave catheter that interfit with each other. FIG. 2C depicts one variation of three single-layer electrodes (i.e., two electrode pairs) that have multiple spark gaps per electrode pair.

FIG. 3A is a perspective view of one variation of a shock wave catheter (balloon not shown). FIG. 3B is perspective view of the single-layer electrodes of the shock wave catheter of FIG. 3A. FIG. 3C is a close-up view of three single-layer electrodes of the shock wave catheter of FIG. 3A.

FIG. 4A is a table that depicts experimental data that reflects the sonic output of a shock wave catheter similar to the shock wave catheter depicted in FIGS. 3A-3C, where the width of the spark gap is 0.003 inch. FIG. 4B is a table that depicts experimental data that reflects the sonic output of a shock wave catheter similar to the shock wave catheter depicted in FIGS. 3A-3C, where the width of the spark gap is 0.004 inch.

FIG. 5 depicts a prospective view of one variation of an electrode pair comprising a band.

FIG. 6 depicts a prospect view of one variation of an electrode pair.

DETAILED DESCRIPTION

Described herein are electrodes for the generation of shock waves within vascular structures. One variation of a shock wave electrode pair comprises a first electrode that is circumferentially disposed over an outer surface of an elongate member and a second electrode also circumferentially disposed over the outer surface of the elongate member, where a spark gap may be formed at the narrowest separation distance between the two electrodes. The electrodes may be planar electrodes that are coplanar with each other (e.g., located along a single layer) over the outer surface of the elongate member (e.g., catheter). In some variations, the first electrode may have a recess (or protrusion) that corresponds with a protrusion (or groove) of the second electrode. The separation between the edge of the recess (or protrusion) of the first electrode and the edge of the protrusion (or recess) of the second electrode may be the shortest distance between the first and second electrodes, and form the spark gap. Multiple pairs of these coplanar or single-layer electrodes may be arranged in series along the surface of an elongate member. In a shock wave catheter system, a voltage generator comprising a positive terminal and a negative terminal may be provided, and a first wire may connect the proximal-most electrode with the positive terminal and a second wire may connect the distal-most electrode with the negative terminal, without the need for additional interconnecting wires between the electrode pairs. Reducing the number of wires that extend along the length of the elongate member may help to maintain the flexibility and steerability of the overall shock wave catheter system, which may facilitate the navigation of the shock wave catheter within tortuous vascular pathways. Reducing the number of wires along the length of the elongate member may also help reduce the thickness or diameter of the overall shock wave catheter. More generally, shock wave catheter systems comprising the single-layer electrodes described herein (i.e., where the surfaces of the first and second electrodes are coplanar) may have a reduced thickness or diameter as compared to shock wave catheter systems comprising stacked multi-layer electrodes (e.g., where the first electrode is on a first layer, the second electrode is on a second layer that is stacked over the first layer and an insulating layer that separates the first and second electrodes). For example, the diameter or thickness of an elongate member with single-layer electrodes may be about 0.025 inch or less, while the diameter or thickness of an elongate member with multi-layer electrodes may be about 0.032 inch or less. Some shock wave catheter systems comprise an angioplasty balloon having an inflated diameter of about 2 mm or less, and single-layer shock wave electrodes may be better suited for this size scale than multi-layer electrodes. In multi-layer electrode designs, shock waves are initiated by plasma arcs that extend across the insulating layer between the electrodes. However, for single-layer electrodes, the plasma arc extends across the spark gap between the electrodes along the surface of the elongate member, thereby eliminating the need for an additional insulating layer. Reducing the overall thickness of the electrode assembly and/or diameter of the shock wave catheter may allow the catheter to be navigated to smaller vascular structures for treatment. For example, shock wave catheters having single-layer electrodes may be more readily inserted in the coronary arteries than shock wave catheters having multi-layer electrodes.

FIG. 1A depicts an example of a shock wave catheter that may be advanced into a patient's vasculature. A shock wave catheter 100 may comprise an elongate member 102, a guide wire lumen 104, a tube 106, and one or more pairs of electrodes 108 enclosed within the tube 106. For procedures in vessels, the tube can be the form of an inflatable, angioplasty balloon. For valvuloplasty application, balloon may not be required. In some variations, a single electrode may be part of two electrode pairs. For example, the proximal end 105 of electrode 107 is part of electrode pair 108 a and the distal end 109 of electrode 107 is part of electrode pair 108 b. The balloon 106 may be collapsed while the shock wave catheter 100 navigates through the vasculature, and expanded (as shown in FIG. 1A) after the catheter is located at the desired treatment position. The catheter may comprise a fluid lumen (not shown) that is in communication with a fluid source that introduces fluid into the balloon 106. The shock wave catheter system may also comprise a voltage generator having a positive terminal and a negative terminal, and a first wire that connects that proximal-most electrode to the positive terminal and a second wire that connects the distal-most electrode to the negative terminal (of course, the polarity may be reversed). After the balloon has been expanded with a fluid to a certain pressure, a voltage pulse may be applied to the electrodes, thereby generating one or more shock waves that may propagate through the fluid and the balloon wall to impinge on a calcification in contact with the balloon. Shock waves may be generated repeatedly, as may be desirable by the practitioner. Although the shock wave catheter 100 is depicted as having four electrode pairs (e.g., electrode pairs 108 a-d), it should be understood that other variations of shock wave catheters may have a different number of electrode pairs (e.g., 1, 2, 4, 5, 7, 8, 10, 12, 16, 20, etc.). In the description of shock wave catheters and electrodes below, a fluid-filled balloon is not depicted, though such a balloon may be included in any of the variations described herein.

In some variations, a single-layer shock wave electrode pair may comprise a first electrode comprising a protrusion and a second electrode comprising a recess that receives the protrusion, where a separation between the edge of the protrusion and the edge of the recess forms a spark gap. For example, the first electrode may comprise a recess and the second electrode may comprise a protrusion that is received by the recess such that the separation between the protrusion and the recess forms a spark gap. A spark gap is a separation between two electrodes across which a plasma arc is likely to form in the presence of a high voltage pulse across those electrodes. The protrusion and the corresponding recess may have any suitable geometry or shape, and may be, for example, shaped like a circle, oval, ellipse, square, hexagon, octagon, triangle, and the like. Protrusions and recesses may have corresponding arcuate shapes or curves. In some variations, the shape of the protrusion and the recess may be selected such that the separation between the first and second electrode is fairly uniform. For example, the protrusion may be circular, so that the distance between the edge of the circular protrusion to the edge of the recess in the second electrode that receives that protrusion may be substantially uniform. The shape of the protrusion and the recess may be selected such that the likelihood of a spark or arc forming at any location along the length of the spark gap is substantially the same. In some variations, the protrusion and the recess may be configured such that the likelihood of a spark forming between the protrusion and the recess is substantially uniform or equal along the length of the spark gap. For example, the protrusion and the corresponding recess may have a smooth contour (i.e., without acute angles, tight turns, or small radii of curvature) such as an arcuate or rounded curve. Arranging the electrodes such that the location of the spark along the spark gap is randomized may help to extend the life of the electrodes as compared to electrodes where the spark always occurs at the same location or region of the spark gap. By arranging the electrodes such that sparks originate at different locations along the gap, the wear on the electrode may be distributed along the gap instead of wearing down a single location along the gap. This may help to lengthen the life of the electrodes as compared to electrodes where sparks originate at the same location or region of the spark gap.

Some electrodes may have one protrusion on one side and one recess on another side (e.g., a protrusion on the proximal edge of the electrode and/or at a first radial position, a recess on the distal edge of the electrode and/or second radial position), and/or a first protrusion on one side and a second protrusion on another side (e.g., a first protrusion on the proximal edge of the electrode and/or at a first radial position, a second protrusion on the distal edge of the electrode and/or at a second radial position), and/or a first recess on one side and a second recess on another side (e.g., a first recess on the proximal edge of the electrode and/or at a first radial position, a second recess on the distal edge of the electrode and/or at a second radial position). In an electrode pair, the first electrode may comprise any number or combination of protrusions and/or recesses (such as those described above) while the second electrode may comprise a corresponding number or combination of recesses and/or protrusions that are complementary to the protrusions and/or recesses of the first electrode. In some variations where the first and second electrodes have more than one pair of complementary protrusions and/or recesses between them, a spark or arc may form between only one of the protrusion/recess pairs at a time (e.g., per voltage pulse), and there may be some variability as to which of the protrusion/recess pairs will spark at a particular time. That is, the spark or arc will only happen at one of the protrusion/recess pairs, while the next spark or arc may be at another one of the protrusion/recess pairs. This variability may help to distribute the wear across the multiple protrusion/recess pairs so that the overall life and/or durability of the electrode pair is extended as compared to an electrode pair where all of the sparks are formed across the same protrusion/recess pair. In some variations, the distal and/or proximal edges of the first electrode and the proximal and/or distal edges from the second electrode may have multiple undulating curves, lobes, peaks and troughs, such that the interface between them comprises a space (which may be a spark gap) that curves between the edges of the electrodes. The space between the two electrodes may have varying distances, which may in turn determine where a spark or plasma arc extends between the electrodes during the generation of a shock wave. For example, to reduce the likelihood that a spark occurs at a particular location between the two electrodes, the spacing at that location may be greater than the spacing in the surrounding areas. To increase the likelihood that a spark occurs at a particular location between the two electrodes, the spacing at that location may be less than the spacing in the surrounding areas. Examples of electrode pairs with varying degrees of separation are further described below.

In some variations, the electrodes may be coated with an insulating material in certain regions and not coated with insulating material (i.e., electrically exposed) in other regions. The location of the insulated regions and exposed regions may also help to increase the likelihood of generating a plasma arc in certain regions while decreasing the likelihood of generating a plasma arc in other regions. For example, insulating the region of the electrodes where the separation between the electrodes is narrow (e.g., may be the narrowest separation) may help to greatly reduce the likelihood of generating a plasma arc across the separation in this region, while exposing (i.e., not insulating) this region may increase the likelihood of generating a plasma arc. The location of a spark gap may be determined at least in part by the relative locations of insulated regions and exposed regions of the electrodes, as well as the size of the spacing/separation between the electrodes at the exposed regions. The location of spark gaps and the characteristics of the shock waves produced by the plasma arcs that span those spark gaps may be determined at least in part by the size, shape and location of the exposed regions of the electrodes. FIG. 1B depicts one variation of an electrode that may be used in any of the shock wave catheters described herein. Electrode 120 may be shaped as a cylindrical band configured to be disposed over the surface of an elongate member. Electrode 120 may comprise a sleeve 122 with a lumen 124 therethrough and a protrusion 126 extending from the sleeve 122. The protrusion 126 may have any of the shapes described and depicted herein, and in the variation of FIG. 1B, may comprise a stem portion 128 and a lobe 130 at the end of the stem. In this example, the shaded region of the lobe 130 may be exposed while the unshaded regions of the electrode 120 are covered by an insulating material. The lobe 130 may be the region of the electrode that interfaces with a recess of a second electrode that has exposed edges (e.g., may be substantially or entirely exposed), and the separation/spacing between the lobe and the edges of the recess may form a spark gap. Exposed or uninsulated regions of two electrodes in close proximity to each other may form a spark gap, regardless of the geometry of the electrodes. Optionally, the exposed regions of the electrodes may be treated (e.g., coated) to help enhance heat dissipation capabilities. For example, the exposed regions of any of the electrodes described herein may have a silver or gold coating. In the variations of shock wave catheters described below, the protrusions or recesses of one electrode and the complementary recesses or protrusions of an adjacent electrode that interfaces with the first electrode may have exposed regions of electrically conductive material to form a spark gap at those interfaces. Teflon, kapton, varnish or oxides and anodized insulations are just a few examples of many suitable insulation materials.

The relative surface area of the exposed regions of an electrode pair may also increase or decrease the likelihood of a spark or arc forming across the spacing/separation between electrodes. For example, the first electrode may have a first exposed region with a first surface area and the second electrode may have a second exposed region with a second surface area, and in some variations, the second surface area may be greater than the first surface area. For example, the ratio between the first surface area and the second surface area may be from about 1:2 to about 1:50, e.g., from about 1:2 to about 1:10, from about 1:4 to about 1:10, from about 1:2 to about 1:20, from about 1:10 to about 1:30, from about 1:20 to about 1:40, from about 1:30 to about 1:50. For example, the area of the first surface area (e.g., of the electrode with the smaller exposed region) may have a radius of about 0.008 inch, and the ratio between the first surface area and the second surface area may be about 1:4. FIG. 1C depicts one variation of an electrode pair 140 comprising a first electrode 142 having a recess 144 and a second electrode 146 having a protrusion 148 that corresponds with the recess 144. In this example, the recess and protrusion both have arcuate shapes. The first electrode 142 and second electrode 146 may be tubular, each with a lumen 141, 145 therethrough configured to be disposed over the surface of an elongate member such that they are coplanar (e.g., in a single layer). The shaded/patterned portions of the electrodes represent electrically exposed (i.e., uninsulated) regions of the electrodes and the unshaded portions represented electrically insulated regions. While the entire surface of the second electrode 146 may be exposed, a small region 143 of the first electrode 142 located around the edge of the recess may be exposed. The surface of the small region 143 is smaller than the surface area of the second electrode, and the ratio between them may be any of the ratios described above. FIG. 1D depicts another variation of an electrode pair 160 comprising a first electrode 162 having a protrusion 164 and a second electrode 166 having a recess 168 that corresponds with the protrusion 164. In this example, the protrusion and the recess both have arcuate shapes. The first electrode 162 and second electrode 166 may be tubular, each with a lumen 161, 165 therethrough configured to be disposed over the surface of an elongate member such that they are coplanar (e.g., in a single layer). While the entire surface of the second electrode 166 may be exposed, only the protrusion 164 of the first electrode may be exposed. The surface of the protrusion 164 is smaller than the surface area of the second electrode, and the ratio between them may be any of the ratios described above. Other variations with different areas and shapes of insulated and exposed electrode regions are described and depicted herein.

FIG. 2A depicts one example of two electrode pairs, where one electrode (e.g., the middle electrode) is a part of both pairs. The first electrode pair 202 may comprise a first electrode 204 and a second electrode 206, and the second electrode pair 210 may comprise the second electrode 206 and a third electrode 208. The first electrode 204 may be electrically connected to the positive terminal of a voltage generator while the third electrode 208 may be electrically connected to the negative terminal of a voltage generator (e.g., by a wire for each connection). The first, second, and third electrodes may be ring-shaped and have a lumen 217 therethrough, and be disposed over the surface of an elongate body on a single-layer (i.e., the electrodes may be co-planar with each other over the surface of the elongate body). The first electrode 204 (i.e., the proximal electrode) may comprise a protrusion 205 that has a stem and a circular lobe. The second electrode 206 may comprise a recess 207 that is sized and shaped to receive the protrusion 205 such that there is a space or gap 209 between the edge of the protrusion 205 and the edge of the recess 207. The second electrode 206 and third electrode 208 may have a similar interface on the opposite side of the system. That is, the second electrode 206 may have a second recess 211 and the third electrode 208 may have a protrusion 213 that is received by the second recess 211 such that there is a space or gap 215 between them. The protrusion 213 may have a stem and a circular lobe similar in size and shape to the protrusion 205, or may have a different size or shape, as may be desired. The circular lobes of the protrusions 205, 213 and the edges of the recesses that receive the protrusions (recesses 207, 211) may be electrically exposed or conductive, while the remainder of the electrodes may be electrically insulated. In this variation, the entire surface of the second electrode 206 may be exposed or uninsulated. As such, spark gaps may be formed at the interfaces of the protrusions and the recesses. The location of the first protrusion 205 and corresponding recess 207 and the location of the second protrusion 213 and corresponding recess 211 may vary according to the desired initiation location of a shock wave. In this example, the first pair and second pair of protrusions/recesses are located radially opposite to each other, with the first pair located on a proximal edge of the electrode and the second pair located on a distal edge of the electrode. In other variations, the first and second pair may both be located on the proximal side (or the distal side) of the middle electrode 206, but radially opposite each other. In some variations, the first and second pair may be radially offset with respect to each other, where the offset angle may be anywhere from about 30 degrees to about 180 degrees in either direction (clockwise or counterclockwise). In some variations, there may be more than one pair of protrusions/recess between each electrode pair. For example, the first electrode 204 may have an additional protrusion or recess at a different radial location and the second electrode 206 may have an additional corresponding recess or protrusion.

Optionally, different regions of each of the electrodes may be covered by an insulating material while other regions are exposed. For example, the portions of the protrusion 205 and the protrusion 213 that are shaded may be exposed, while the remainder of the electrode 204 and the electrode 208 may be covered by an insulating material. The second electrode 206 may be entirely exposed and uninsulated. Alternatively, at least the regions around the edges of the recesses 207 and 211 may be exposed, while the remainder of the electrode may be insulated. The exposed regions may optionally have a silver or gold coating. As described previously, the relative sizing of the surface area of the exposed regions between the electrodes in a pair may help to facilitate and guide the electric current flow between electrodes so that plasma arcs or sparks occur at the desired spark gap location. In some variations, the likelihood of creating a plasma arc that is capable of generating a shock wave is increased when the surface area of the exposed (i.e., uninsulated) region of a first electrode is smaller than the surface area of the exposed region of a second electrode that is adjacent to it. The exposed surface area differential may be represented by the ratio of the surface area of an exposed region of a first electrode to the surface area of an exposed region of a second electrode. The interface between the electrodes of a pair described in any of the shock wave catheters disclosed herein, regardless of their shape or location, may have the exposed surface area differential described above.

FIG. 2B depicts another variation of two electrode pairs that are similar to the electrode pairs depicted in FIG. 2A, except that where there was a protrusion, there is now a recess and where there was a recess, there is now a protrusion. For example, the first electrode pair 222 comprises a first (proximal) electrode 224 and a second (middle) electrode 226. The first electrode 224 comprises a recess 225. The second (middle) electrode 226 comprising a protrusion 227 that is located within the recess 225. The second electrode pair 230 comprises the second electrode 226 and a third (distal) electrode 228. The second electrode 226 comprises a second protrusion 231. The third (distal) electrode 228 comprises a recess 233 within which the protrusion 231 is located. In other words, instead of the middle electrode having two radially offset and opposite recesses as illustrated in FIG. 2A, the middle electrode now has two radially offset and opposite protrusions as illustrated in FIG. 2B. The first electrode 224 may be connected to the positive terminal of a voltage generator while the third electrode 228 may be connected to the negative terminal of a voltage generator (e.g., by a wire for each connection). The various parameters and variants described above for FIG. 2A may also be applicable in the variation depicted in FIG. 2B.

FIG. 2C depicts an example of a shock wave catheter 250 comprising a first electrode 252, second electrode 254, and third electrode 256 (for clarity, the elongate body, guide wire lumen, and wiring of the catheter 250 are not shown). The first, second and third electrodes are coplanar with each other, arranged on a single layer along the surface of the elongate body of the catheter. The first electrode 252 may comprise a first recess 258 and a first protrusion 260. The second electrode 254 may comprise a second protrusion 262 a, a second recess 264 a, a third protrusion 262 b opposite to the second protrusion 262 a, and a third recess 264 b opposite to the second recess 264 a. The third electrode 256 may comprise a fourth recess 266 and a fourth protrusion 268. There may be two spark gaps located between the first and second electrodes, and two other spark gaps located between the second and third electrodes. In some variations, of the two spark gaps between an electrode pair, only one spark gap will form a plasma arc during a voltage pulse. The location and arcuate curvature of the first recess 258 may correspond with the location and arcuate curvature of the second protrusion 262 a to form a first spark gap 251 therebetween, and the location and arcuate curvature of the first protrusion 260 may correspond with the location and arcuate curvature of the second recess 264 a to form a second spark gap 253 therebetween. Similarly, the location and arcuate curvature of the third recess 264 b may correspond with the location and arcuate curvature of the fourth protrusion 268 to form a third spark gap 255 therebetween, and the location and arcuate curvature of the third protrusion 262 b may correspond with the location and arcuate curvature of the fourth recess 266 to form a fourth spark gap therebetween (not visible in this view). While it may be desirable that some regions of the separation between the electrodes (i.e., the protrusions and recesses) form spark gaps where the likelihood of plasma arc formation is relatively high, there may be some electrode separation regions where it is desirable for the likelihood of plasma arc formation to be relatively low. One way of reducing the likelihood of forming a plasma arc at a particular separation region is to increase the distance between the two electrodes relative to the surrounding separation region. An example is depicted in FIG. 2C. The first electrode 252 comprises a recess or groove 270 and the second electrode 254 comprises a recess or groove 272 that is aligned with the groove 270 of the first electrode 252. Aligning two recesses or grooves may increase the width of the separation between the first electrode 252 and second electrode 254 relative to the width of the separation in the intended spark gap regions. In some other variations, a first electrode may have a recess while the second electrode may have a straight edge (e.g., no protrusion) in the region of the recess. While the recesses are depicted as having an arcuate or curved shape, it should be understood that the recesses may have any shape. For example, a recess may have straight edges (e.g., a square shape, rectangular shape, triangular shape, etc.), and/or have curved edges (e.g., circle, oval, ellipse, semi-circle, semi-oval, semi-ellipse, etc.), and/or a combination of straight and curved edges (e.g., rectangular, triangular or any polygon with rounded corners and/or undulating edges). The increased separation between the electrodes provided by one or more recesses may help to reduce the likelihood of formation of a plasma arc at the recessed region(s). In some variations, one or both of the electrodes in a pair may have edge(s) that curve away from the edge of the other electrode. For example, the electrodes may have an undulating curved edge (comprising one or more concave or convex curves) where the peaks and troughs do not follow each other (e.g., are out-of-phase with each other). For example, one electrode may have a straight edge (i.e., without a recess or protrusion), while the other electrode may have undulating curves comprising a convex curve at a desired spark gap region and a concave curve in regions where no spark gap is desired. Alternatively or additionally to increasing the separation width in a region where no plasma arc is desired, such regions may be electrically insulated, which may also help to impede the formation of a plasma arc.

One variation of a shock wave catheter system having one or more single-layer electrodes is depicted in FIGS. 3A-3D. The shock wave catheter 300 may comprise an elongate member 302, a guide wire lumen 304, and five electrodes 306 a,b,c,d,e that form four electrode pairs 308 a,b,c,d. The shock wave catheter 300 may also comprise a balloon (e.g., an angioplasty balloon) that encloses the electrodes (not shown), a first wire that connects the proximal-most electrode 306 a to a positive terminal of a voltage source and a second wire that connects the distal-most electrode 306 e to a negative terminal of a voltage source. As previously described, the balloon is filled with a fluid medium before a voltage is applied across the electrodes for the generation of shock waves. The first and second wires may each extend along the surface and/or lumen and/or within the wall of the elongate member 302. In some variations, the wires may extend along interior walls of the guide wire lumen 304. Each electrode pair may have one or more spark gaps as may be desirable. For example, a first electrode pair may have one spark gap capable of initiating one shock wave, while a second electrode pair may have two spark gaps capable of initiating two shock waves (i.e., one shock wave per spark gap). In other examples, an electrode pair may have more than two spark gaps, and may have three, four, five or more spark gaps. When a voltage is applied across the proximal-most electrode 306 a and the distal-most electrode 306 e, a series of plasma arcs may form serially across the spark gaps between the electrodes (i.e., from electrode 306 a to electrode 306 b, from electrode 306 b to electrode 306 c, from electrode 306 c to electrode 306 d, from electrode 306 d to electrode 306 e, which then guides the current back to the negative terminal of the voltage source via the wire) to initiate a series of expanding shock waves. The number of initiated shock waves may correspond to the number of spark gaps between the electrodes (e.g., each spark gap gives rise to one plasma arc per voltage pulse and/or each plasma arc initiates one shock wave), and/or may be greater than (e.g., a spark gap may give rise to more than one plasma arc per voltage pulse, and/or each plasma arc initiates one or more shock waves) or less than (e.g., plasma arcs may not form across all of the spark gaps) the number of spark gaps. FIG. 3B depicts the electrodes 306 a,b,c,d,e without the elongate member 302. The length of each electrode with respect to each other may vary. For example, the proximal-most electrode 306 a and the distal-most electrode 306 e may be shorter than the second electrode 306 b and fourth electrode 306 d. The shortest electrode may be the center electrode 306 c. Electrodes whose lengths extend along a substantial segment of the elongate member may be configured such that their effect on the flexibility and bendability of the elongate member is reduced. For example, electrode 306 b (as well as electrode 306 d) may comprise a proximal portion (e.g., a band) 310, a distal portion (e.g., a band) 312, and a body portion 314 extending between the proximal and distal portions. In this variation, the proximal and distal portions may be generally cylindrical or marker band-like structures. The body portion may be covered by an insulating material, which may help to facilitate current flow between the proximal and distal portions along the body portion. The structure of the body portion 314 may be selected to help facilitate bending of the electrode 306 b as the elongate body 302 bends. In the variation depicted in FIGS. 3A-3C, the body portion 314 may comprise a helical structure or spiral that wraps around the surface of the elongate body 302 between the proximal and distal portions. The threads of the helical structure or spiral of the body portion 314 may be selected in order to accommodate flexion, torquing, and/or steering of the shock wave catheter. For example, the body portion 314 may have a pair of spirals (e.g., a double-helix), where each spiral has one twist. In other variations, there may be more spirals (e.g., triple or quadruple helices), and/or each spiral may have more than one twist (e.g., two, three, four, five or more twists).

FIG. 3C is a close view of the interface between electrodes 306 b,c,d. In this variation, electrode 306 b has a recess 320 that corresponds to a protrusion 322 on electrode 306 c (which form electrode pair 308 b), and electrode 306 c has a recess 324 that corresponds to protrusion 326 on electrode 306 d (which form electrode pair 308 d). There may be a separation 323 between electrodes 306 b and 306 c, where the narrowest portion of the separation 323 may be a spark gap 325. In this variation, the protrusions 322, 326 may have a circular lobular portion 328 connected to a stem 330. The stem 330 may extend between the widest part of the separation 323 between the electrodes, while the perimeter of the circular lobular portion 328 may comprise at least a portion of the spark gap 323. In this example, the protrusions and recesses for both electrode pairs have the same or similar shape and size, however, in other variations, the protrusions and recesses may have different shapes or sizes.

The electrodes 306 b,c,d may have electrically insulated regions and exposed (i.e., electrically non-insulated) regions, the location and sizing of which may be configured to help guide the direction of the current flow and/or to facilitate the formation of plasma arcs or sparks at the desired spark gap locations. In the variation depicted in FIG. 3C, the majority of the surface area of the electrode 306 b may be exposed or uninsulated, while the majority of the surface area of the electrode 306 c is insulated, except for the regions that are adjacent to the spark gap 325. That is, the protrusion 322 (e.g., the circular lobe of the protrusion) and the region along the edge of the recess 324 of the electrode 306 c may be exposed, but the remainder of the electrode 306 c may be insulated (e.g., the stem portion of the protrusion, the cylindrical region or body of the electrode extending between the proximal and distal portions or bands of the electrode, the spiral or twisted region of the body, the body region between the proximal and distal portions or bands of the electrode, etc.). The circular lobular portion 328 of the electrode 306 d may be exposed, while the stem portion 328 and the body of the electrode (e.g., the portion extending between the proximal and distal ends) may be insulated. The surface area of the exposed portion the electrode 306 b may be larger than the surface area of the protrusion 322 that is exposed. For example, the ratio between the surface area of the exposed portion may be from about 1:2 to about 1:50, e.g., from about 1:2 to about 1:10, from about 1:2 to about 1:20, from about 1:10 to about 1:30, from about 1:20 to about 1:40, from about 1:30 to about 1:50. Similarly, the surface area of the circular lobular portion 328 may be smaller than the surface area of the exposed region along the edge of recess 324, and may have similar ratio values described above. Some variations of electrodes may have a proximal portion with an exposed protrusion or recess edge, a distal portion with another exposed protrusion or recess edge, and a body portion between the proximal and distal portions that is insulated. The surface area of the one or more exposed regions of an electrode may affect the strength, shape, location, etc. of the plasma arc formed, which in turn affects the sonic output and/or direction of the initiated shock wave.

Experimental Data and Results

An experiment was conducted with a prototype of the shock wave catheter depicted and described in FIGS. 3A-3C. FIGS. 4A and 4B are tables that represent the sonic output of shock wave generated across the electrodes depicted in FIGS. 3A-3C. In this experiment, the shock wave electrodes were located in the center of a 2.5 mm×25 mm L2140 balloon filled with saline solution at a pressure of 4 ATM. The magnitude of the generated sonic output was measured 2 microseconds from the initiation time of the plasma arc or spark. The propagation distance of a shock wave at this time delay is about 3 mm. The voltage pulse width was 1 microsecond, with a magnitude of 3000 volts. The diameter of the wires that connected the electrodes to the positive and negative terminals of the voltage source was 0.005 inch and the length was about 170 cm. Insulated portions of the wire were covered with a polyimide insulation layer with a thickness of 0.0005 inch. Two spark gap sizes were tested by measuring the sonic output of the generated shock waves. The table of FIG. 4A depicts sonic output data from an electrode pair with a spark gap of about 0.003 inch. The table of FIG. 4B depicts sonic output data from an electrode pair with a spark gap of about 0.004 inch. Based on this set of experiments, a smaller spark gap (e.g., with a smaller width) may give rise to shock waves with higher sonic output than a larger spark gap.

FIG. 5 depicts a prospective view of an electrode pair comprising a band. As shown in FIG. 5, in one variation, an electrode pair 500 may comprise a first electrode and a second electrode. The first electrode may comprise band 520. The second electrode may comprise a portion of a wire 512 that is exposed or uninsulated. In one variation, the wire 502 may be electrically coupled to a positive terminal of a voltage source, and the wire 512 may be electrically coupled to a negative terminal of the voltage source, or vice versa. The wire portion 506 (e.g., a portion of the wire 502) and the wire portion 510 (e.g., a portion of the wire 512) may be coupled using a band 520. The band 520 may comprise one or more grooves, such as a first groove 504 and a second groove 508. In some variations, the first groove 504 may have a dimension (e.g., width) that is smaller than the corresponding dimension of the second groove 508. As a result, the wire portion 506 may be snuggly disposed in the first groove 504 and the wire portion 510 may be loosely disposed in the second groove 508. The wire portion 510 may be loosely disposed in groove 508 to prevent shorted circuit between the wire portion 510 and the band 520. This feature also maintains the spark gap as the wire portion 510 erodes away.

As shown in FIG. 5, the wire 502 may comprise the wire portion 506 that is disposed in the first groove 504. In some variations, the wire 502 is insulated with an uninsulated or exposed section (e.g., wire portion 506). The wire portion 506, which is exposed or uninsulated, may be snuggly disposed in groove 504 to have a good electrical connection between the wire 502 and the band 520 (no spark in this region). The band 520 thus forms the first electrode. The wire 512 may comprise the wire portion 510 disposed in the second groove 508 of band 520. In some variations, the wire 512 and the wire portion 510 are insulated, and insulation is removed at the end or tip of the wire portion 510 to form a spark gap 514. The exposed or uninsulated portion of wire portion 510 thus forms the second electrode. The spark gap 514 separates the first electrode (e.g., band 520) and the second electrode (e.g., the exposed or uninsulated end or tip of wire portion 510). As described, the surface area of the one or more exposed regions of the first and second electrodes may affect the strength, shape, location, etc. of the plasma arc formed, which in turn affects the sonic output and/or direction of the initiated shock wave. The increase or decrease of the separation between the first and second electrodes may also increase or decrease the likelihood of formation of a plasma arc at the spark gap.

FIG. 6 depicts a prospective view of one variation of an electrode pair. As shown in FIG. 6, in one variation, an electrode pair 600 comprises a first electrode 602 and a second electrode 604. The first electrode 602 and the second electrode 604 may be shaped as a cylindrical band configured to be disposed over the surface of an elongate member. The first electrode 602 may comprise a protrusion (e.g., a tongue) 606; and the second electrode 604 may comprise a recess (e.g., a groove) 608. As shown in FIG. 6, the protrusion 606 and the recess 608 may be oriented perpendicular to the axis of the first electrode 602 and the second electrode 604, respectively. The protrusion 606 and the recess 608 may have corresponding arcuate shapes or curves. The protrusion 606 and the corresponding recess 608 may have any suitable geometry or shape, and may be, for example, shaped like a circle, oval, ellipse, square, hexagon, octagon, triangle, and the like.

In some variations, the first electrode 602 may be electrically coupled to a positive terminal of a voltage source, and the second electrode 604 may be electrically coupled to a negative terminal of the voltage source, or vice versa. In one variation, the surface area of the protrusion 606 (e.g., a tongue) may be electrically insulated using a polyimide coat except the end or tip region of the protrusion 606. The surface area of recess 608 may also be electrically insulated using a polyimide coat except the end region of the recess 608 (e.g., the groove area). Thus, the end or tip region of the protrusion 606 and the end region of the recess 608 are exposed to enable the generating of a spark. As described, the surface area of the one or more exposed regions of the first and second electrodes may affect the strength, shape, location, etc. of the plasma arc formed, which in turn affects the sonic output and/or direction of the initiated shock wave. The increase or decrease of the separation between the first and second electrodes may also increase or decrease the likelihood of formation of a plasma arc at the spark gap. In one variation, the insulation (e.g., the polyimide coating) of the protrusion 606 and the recess 608 may erode away after repeated spark generation. The erosion of the insulation may expose additional areas and thus increase or maintain the spark generation.

It will be understood that the foregoing is only illustrative of the principles of the invention, and that various modifications, alterations and combinations can be made by those skilled in the art without departing from the scope and spirit of the invention. Any of the variations of the various shockwave devices disclosed herein can include features described by any other shockwave devices or combination of shockwave devices herein. Furthermore, any of the methods can be used with any of the shockwave devices disclosed. Accordingly, it is not intended that the invention be limited, except as by the appended claims. For all of the variations described above, the steps of the methods need not be performed sequentially. 

What is claimed is:
 1. A shock wave catheter comprising: an axially extending elongate member; a first ring electrode circumferentially disposed over an outer surface of the elongate member, the first ring electrode including one of a first recess or a first projection located along a first edge of the first ring electrode; a second ring electrode circumferentially disposed over the outer surface of the elongate member and longitudinally adjacent to the first ring electrode, the second ring electrode including the other of said first recess or the first projection along a first edge of the second ring electrode that is adjacent the first edge of the first ring electrode, with the first projection being received by the first recess, wherein a first spark gap is formed by a separation between the first projection and the first recess, and wherein the opposing edge of the second ring electrode includes one of a second recess or a second projection; a third ring electrode circumferentially disposed over the outer surface of the elongate member and longitudinally adjacent the second ring electrode, the third ring electrode including the other of said second recess or second projection along an edge thereof, with the second projection being received in the second recess wherein a second spark gap is formed by a separation between the second projection and the second recess, and wherein each recess and the projection associated therewith are coplanar; and a first wire electrically coupled to the first ring electrode and a second wire electrically coupled to the third ring electrode and with the ring electrodes being configured such that when a voltage pulse is applied across the first and second wires, a current flows across the first and second spark gaps creating a series connection between the first, second and third ring electrodes and initiating shock waves at the spark gaps.
 2. The shock wave catheter of claim 1, further comprising a voltage source, wherein the first wire connects the first electrode to a positive terminal of the voltage source, and the second wire connects the second electrode to a negative terminal of the voltage source.
 3. The shock wave catheter of claim 1, wherein the axially extending elongate member comprises a guide wire lumen extending therethrough.
 4. The shock wave catheter of claim 1, wherein the first and second recesses have a concave curvature and the first and second projections have a convex curvature that corresponds with the concave curvature.
 5. The shock wave catheter of claim 1, wherein each of the electrodes comprises a proximal end, a distal end, and a spiral body therebetween.
 6. The shock wave catheter of claim 5, wherein the spiral body comprises one or more helices that wrap along the outer surface of the elongate body.
 7. The catheter as recited in claim 1, wherein the spacing between each recess and the projection associated therewith is less than the spacing between the edges of the ring electrodes.
 8. The catheter as recited in claim 1 further including a tube connected to the elongate member and surround the ring electrodes, said tube being filled with a conductive liquid.
 9. A shock wave catheter comprising: an axially extending elongate member; a first ring electrode mounted circumferentially around the elongate member; a second ring electrode mounted circumferentially around the elongate member and positioned longitudinally adjacent to the first ring electrode; a third ring electrode mounted circumferentially around the elongate member and positioned longitudinally adjacent to the second ring electrode, and wherein a first spark gap is defined between the first and second ring electrodes and a second spark gap is defined between the second and third ring electrodes, each spark gap including an arcuate recess formed along a side edge of one of the ring electrodes and a complementary arcuate projection formed along the side edge of the adjacent ring electrode, the projection fitting into the associated recess and wherein the first spark gap is circumferentially offset from the second spark gap, and wherein each recess and the projection associated therewith are coplanar; and a first wire electrically coupled to the first ring electrode and a second wire electrically coupled to the third ring electrode and with the ring electrodes being configured such that when a voltage pulse is applied across the first and second wires, a current flows across the first and second spark gaps creating a series connection between the first, second and third ring electrodes and initiating shock waves at the spark gaps.
 10. The catheter as recited claim 9, wherein the second ring electrode is a non-insulated metal and wherein the recess of the first and second spark gaps are formed in the second ring electrode and wherein the projection of the first spark gap is formed in the first ring electrode and the projection of the second spark gap is formed in the third ring electrode and wherein the first and second ring electrodes have an insulating coating except in the region of the projections.
 11. The catheter as recited in claim 10, wherein the surface area of the second ring electrode is greater than the surface area of each of the projections of the first and third ring electrodes.
 12. The catheter as recited in claim 9, wherein the first electrode and the third electrode each have a proximal portion, a distal portion, and a body therebetween, wherein the body extends longitudinally along the axially extending elongate member and comprises a spiral.
 13. The catheter as recited in claim 12, wherein the body comprises a helix.
 14. The catheter as recited in claim 12, wherein the body comprises one or more turns around the surface of the elongate member.
 15. The catheter as recited in claim 9 further comprising: a fourth ring electrode mounted circumferentially around the elongate member and positioned longitudinally adjacent to the third ring electrode, and wherein a third spark gap is defined between the third and the fourth ring electrodes and includes a recess and a complementary projection, and wherein the second wire is electrically coupled to the fourth ring electrode.
 16. The catheter as recited in claim 9, wherein the spacing between each recess and the projection associated therewith is less than the spacing between the edges of the ring electrodes.
 17. The catheter as recited in claim 9 further including a tube connected to the elongate member and surround the ring electrodes, said tube being filled with a conductive liquid. 